A studded tire from which metal made pins are projected for use in the winter season, and a studless tire wherein special grooves are formed in its tread surfaces have been frequently used.
However, the use of studded tires has been recently prohibited due to the dust problems, road wear problems and the like. Today, therefore, various makers have tried to develop tires which are provided with means for increasing the frictional coefficient on a water soaked or snowy road.
For instance, it has been proposed to use tires in which pulverized materials, such as broken egg shells or small pieces of walnuts, are embedded in the tread surface portion, as shown in Japanese Patent Application Laid-Open No. Hei 6-41355.
However, these tires suffer from problems that the embedded materials such as pulverized eggs, metal pieces and walnut pieces would fall apart from the tread surface portions. As a result, of course, the slippage preventing function would deteriorate.
As a result of elaborate studies made by the present inventor, it has been found that the reason why the pulverized egg shell pieces would fall apart from the tread surface portion of the tires would be based upon the fact that the bonding between the pulverized pieces and the rubber materials become worse and the rubber material becomes hard in particular at a low temperature. In order to solve these problems, the present inventor developed the present invention.